If you want to compare two revisions in an item's history, for example revisions 100 and 200 of the same file, just use → to list the revision history for that file. Pick the two revisions you want to compare then use → .
If you want to compare the same item in two different trees, for example the trunk and a branch, you can use the repository browser to open up both trees, select the file in both places, then use → .
If you want to compare two trees to see what has changed, for example the trunk and a tagged release, you can use → Select the two nodes to compare, then use → . This will show a list of changed files, and you can then select individual files to view the changes in detail. You can also export a tree structure containing all the changed files, or simply a list of all changed files. Read Section 4.10.3, “Comparing Folders” for more information. Alternatively use → to see a summary of all differences, with minimal context.